Phenolic resins are a class of plastics that have balanced properties, and which hence are extensively used as molding materials, binders, adhesives, and in many other applications. Phenolic resins are generally classified as novolaks and resoles.
Novolaks are resins that are prepared by thermal reaction, in the presence of an acid catalyst such as oxalic acid or hydrochloric acid, between a phenol and an aldehyde, with the aldehyde-to-phenol molar ratio being set at about from 0.7/1 to about 0.9/1. Molecules of a novolak resin are characterized not only by a structure wherein from 4 to 8 phenol molecules are linked in a straight chain by methylene bonds, but also by methylol groups, which are far smaller in number than in the case of a resole resin. Because of the low content of methylol groups, the novolak resin will not cure by simple heating. Therefore, if the novolak resin is to be used in a cured state, it is usually cured by heating in the presence of from about 8 to about 15 wt% of a curing agent such as hexamethylenetetramine.
Resoles, on the other hand, are the resins that are prepared by reacting a phenol with from about 1.1 to about 1.5 molar excess of an aldehyde in the presence of a basic compound such as aqueous ammonia or sodium hydroxide. The molecule of the resole resin is characterized by a branched structure and the presence of many methylol groups. Becase of the high methylol content, the resole resin will cure simply by heating.
The differences in properties between novolak and resole resins are reflected both in their thermal curing characteristics during the reaction for curing and in the physical properties of the cured molding; the novolak resin that is cured with hexamethylenetetramine is superior to the resole resin cured by simple heating. However, when hexamethylenetetramine is used as a curing agent, it will decompose to evolve ammonia or formaldehyde gas, which causes voids in the shaped article of the cured novolak. In addition, any unreacted hexamethylenetetramine will remain in the shaped novolak to impair its physical properties. Furthermore, it is difficult to achieve uniform dispersion of hexamethylenetetramine in the novolak resin so as to produce an evenly cured molding.
With a view to solving these problems while making the most of the advantages of novolak resins, attempts have been made to react the novolak resin with a basic compound such as hexamethylenetetramine, and, optionally, with an aldehyde. However, the reaction rate of this method is so high that the reaction mixture will gel very rapidly and fail to produce the desired thermosetting phenolic resin.
Methods have also been described for producing a resole resin in the form of spherical solid particles. For instance, U.S. Pat. Nos. 3,823,103, 4,026,828, 4,039,525, 4,206,095, 4,182,696, 4,316,827, and 4,366,303 disclose methods for producing microspherical resole resin particles by emulsion-polymerizing phenols and aldehydes in aqueous media in the presence of both basic catalysts and protective colloids such as gum arabic, gum ghatti, hydroxy alkylated guar gum, and partially hydrolyzed PVA (polyvinyl alcohol). Japanese Patent Publication No. 42077/78 shows a method of producing microspherical resole resin particles by first reacting a phenol and formaldehyde in the presence of a nitrogenous compound such as ethylenediamine, then adding a hydrophilic organic polymer, such as gelatin, casein, or PVA, to the resulting condensation product, and containing the reaction. The phenolic resins produced by these methods are thermosetting, but the thermal curing products thereof are not as satisfactory as the hexamethylenetetramine-cured novolak resin in terms of either the thermal curing characteristics or the physical properties of the articles molded from these phenolic resins.